1. Field of the Invention
The present invention relates a member for supporting magnetic disc substrates stacked around a hub for a magnetic disc memory device and to a process of producing such a supporting member for a magnetic disc memory device.
2. Prior Art
Magnetic disc memory devices, which have widely been used in a wide variety of computerized electronic appliances. A conventional magnetic disc memory device is shown in FIG. 3, which includes a hub 5 which is fixed by an driving axis 4 and rotated at high speed, a plurality of magnetic disc substrates 6 which are stacked spaced from each other around the central hub and a plurality of spacers each which is inserted between the adjacent two of the magnetic disc substrates for spacing between the two. A cramp disc also is used to fit on the top end of the hub by screws and fix the topmost magnetic disc substrate.
In magnetic disc memory devices, one or more magnetic heads are disposed movably in space between the adjacent magnetic disc substrates to truck recording magnetic faces formed on the substrates in a non-contact mode, In use, while the magnetic disc substrates are being rotated at a controlled speed, magnetic heads read and write necessary information and data at allotted portions on the recording magnetic faces of the substrates.
Recently, in order to increase recording capacity for the memory devices, higher information density has been required than before and therefore a clearance of a magnetic head relative to a magnetic recording face must be made as small as possible. For this purpose, magnetic recording faces of magnetic memory discs are necessary to have higher flatness and smoothness with substrates prevented from deflecting.
Proposals have been made to form magnetic disc substrates of aluminum, ceramic or glass, and to make members for supporting the magnetic disc substrates such as spacers, shims and cramps out of ceramic. These materials are almost free of temperature change in dimensions and are highly and precisely machinable.
Further, magnetic memory discs are known to often suffer from electrostatic charging, which has a risk of destroying information recorded on the discs. Japanese Patent publication Nos. 09-027177, 09-044969, and 07-098912 have proposed that substrates for magnetic memory discs are fabricated of ceramic sintered materials with electric conductivity or semiconductivity, for example, based on alumina, zirconia, forsterite or steatite. The conductive sinters can release accumulated charges from the surfaces of the discs to earth and protect stored information on the recording faces from destroying.
However, in the case where supporting members made of ceramics are disposed in magnetic disc memory devices such that a clearance between a magnetic head and a magnetic disc is set below 0.1 μm, there is a risk of particles occluding the clearance or making a magnetic head push a disc recording face, which damages information and data having sorted on the recording face. Ceramic sinters are generally of polycrystalline structure which is formed by firing a compacted ceramic powder so that they have some voids or pores opening in the surfaces.
After polishing such ceramic sinters to be made into a desirable shape, cut shavings or polished grains are left within the open pores, which can not perfectly be removed even if the sinters are cleaned and rinsed, for example, using conventional ultrasonic cleaning or ultraviolet ionized cleaning methods. In use of disc devices, such particles are separated apart from the voids due to centrifugal force or vibration and transferred onto the recording faces of the magnetic memory discs, deteriorating the recording faces.